1. Field of the Invention
The present invention relates to a method of adjusting a transmission electron microscope equipped with a monochromator that monochromatizes an electron beam.
2. Description of Related Art
In the past, monochromators for monochromatizing electron beams to achieve higher resolutions in electron energy loss spectroscopy (EELS) implemented in transmission electron microscopy have been known (see, for example, JP-A-2004-327377). Various types of monochromators have been proposed. They are classified into two major categories: single-stage filter type and two-stage filter type.
A monochromator of the single-stage filter type is composed of an energy filter acting to spectrally resolve an electron beam and an energy selecting filter. An electron beam emitted from an electron beam source is spectrally separated by the energy filter and focused onto the face of the energy selecting slit. Since the beam is spectrally separated, a spectrum corresponding to a velocity distribution of the electron beam is formed on the energy selecting slit. The energy selecting slit permits only electrons having a certain velocity to pass through the slit, whereby the electron beam is monochromatized. Because the electron beam transmitted through the energy selecting slit forms a spectrum corresponding to the width of the slit at the focal plane, a virtual light source of the electron beam does not assume a circular form. Consequently, in a scanning transmission electron microscope (STEM) image obtained by focusing the electron beam onto a sample, the shape of the light source that is spread along the spectrum is reflected. Anisotropy in resolution appears. Furthermore, in a transmission electron microscope (TEM) image obtained by illuminating a wide area of a sample with an electron beam, anisotropy appears in coherence of the electron beam because of the shape of the virtual light source that is spread along the spectrum.
A monochromator of the two-stage filter type has two stages of energy filters. An energy selecting filter is positioned between the two stages of energy filters. The first stage of energy filter and the energy selecting filter are configured in the same way as in the monochromator of the single-stage filter type. In the monochromator of the two-stage filter type, the energy dispersion of the electron beam transmitted through the energy selecting slit is nullified by the second stage of energy filter, and the focal plane formed after exiting the monochromator agrees with the achromatic plane. The virtual light source at the focal plane of the electron beam which has been achromatized at this plane assumes a circular form. Consequently, in a STEM image obtained by focusing the electron beam onto a sample, the resolution shows no anisotropy and, thus, it is possible to investigate details of the electronic state of a substance by high resolution electron energy loss spectroscopy at a spatial resolution on the order of nanometers or sub-nanometers. Furthermore, in a TEM image obtained by illuminating a wide area of a sample with an electron beam, isotropic coherence of the electron beam at the virtual light source in the form of a spot is combined with further decreases in the effects of chromatic aberration owing to irradiation by a monochromatized electron beam. Hence, higher resolution imaging is enabled.
The great advantage of the monochromator of the two-stage filter type over the monochromator of the single-stage filter type is that a virtual light source of an electron beam that is monochromatized and achromatized is obtained. To achieve this feature, it is necessary that the optical system for use with the monochromator of the two-stage filter type be set up so as to satisfy two requirements: convergence of the electron beam on the energy selecting slit and achromatization at the focal plane formed after exiting from the monochromator. However, because of the complex structure of the monochromator of the two-stage filter type, achieving these requirements in the optical system of a practical instrument involves difficulties. Especially, it is difficult to judge whether it is possible to make an adjustment successfully on achromatization at the focal plane formed after exiting the monochromator.